This invention relates to projection optical devices which employ reflective LCD""s.
FIG. 1 is an on-axis layout of a prior art projection system employing three reflective LCD panels, i.e., LCD 10 for red light, LCD 12 for green light, and LCD 14 for blue light. Light form a source 16 is polarized by a conventional polarization beam splitting cube 18 and the S-component of the light (shown by a continuous line in this and all other figures) is reflected towards color cube 20 and the LCD""s. The color cube separates light into the red, green, and blue primary colors.
Light reflected from the xe2x80x9conxe2x80x9d pixels of the LCDs has P-polarization (shown by a broken line in this and all other figures) and thus passes through the diagonal of the polarization beam splitting cube (PBS cube) to projection lens 22 to create an image on a screen (not shown). Light reflected from xe2x80x9coffxe2x80x9d pixels has S-polarization, which reflects from the diagonal of the PBS cube and thus does not pass into the screen space.
Although the design of FIG. 1 has a compact architecture, it suffers from a number of fundamental drawbacks, including: (1) P-polarization from the source is completely lost; (2) the contrast of the system is limited by the polarization properties of the diagonal of the PBS cube, which properties tend to be poor for the f-numbers typically required for commercial projectors; and (3) the color cube needs to work with both S and P polarization for all three colors, which leads to low efficiency of color separation/recombination.
In particular, the slope of a dichroic cut-off filter is always less than 100% so that to avoid undesired color mixing, some light on the borders between blue and green and between green and red must be intentionally cut off and thus lost. Moreover, the amount of light cut off must take account of the fact that the slope and cut-off wavelength of a dichroic filter depend on polarization and angle of incidence. As a result of these effects, substantial light loss can occur when all three LCDs are located in the same polarization channel and a single color cube is used to split and recombine the three primary light colors.
Japanese Patent Publication No. 10-253922 illustrates a variation of the system of FIG. 1 which uses two dichroic prisms instead of a single color cube. Although better than the basic system, the approach of this patent publication is mechanically complex and can exhibit reduced contrast through its reliance on a single polarization beam splitting cube having only one polarizing diagonal.
Another known configuration is that developed by S-VISION Inc. of Santa Clara, Calif. See Bone et al., xe2x80x9cNovel Optical System Design for Reflective CMOS Technology,xe2x80x9d Proceedings of SPIE, Volume 3634, pages 80-86, 1999. This approach uses off-axis optics, two color cubes, and two film polarizers (one as a polarizer and the other as an analyzer) to enhance contrast. Each color cube works with one polarization, which increases the efficiency of color separation/recombination. The problems here include: (1) the cost of two color cubes; (2) loss of the second polarization from the light source; and (3) the complexity and expense of off-axis optics.
In view of the foregoing, it is an object of the invention to provide improved optical systems for use with reflective LCD""s. In particular, it is an object of the invention to provide optical systems which have some and preferably all of the following properties: (1) the system is mechanically simple and compact, (2) the system minimizes the use of expensive optical components, (3) the system uses at least some of the light of each polarization, and (4) the system achieves a high level of contrast.
To achieve these and other objects, the invention in accordance with certain of its aspects provides a unitary polarization beam splitter having first, second, third, and fourth sides and two polarizing diagonals, said diagonals intersecting at right angles, said splitter being symmetric about each diagonal (i.e., the splitter has mirror symmetry about each diagonal), wherein light having S polarization primarily reflects from each diagonal and light having P polarization primarily transmits through each diagonal.
In certain preferred embodiments, the unitary splitter comprises four polarization beam splitting cubes, each cube adjoining two other cubes along two faces to form the overall polarization beam splitter. In some embodiments, the cubes are all of the same size, while in other embodiments, two of the cubes have a first size and two of the cubes have a second size.
Half wave plates and/or sheet polarizers can be located between adjoining faces of the polarization beam splitting cubes. Zero, two or four half wave plates and zero, two, or four sheet polarizers can be used in any combination as desired. The sheet polarizers improve contrast. The half wave plates can be used to compensate for skew ray polarization leakage or can be used to improve contrast in which case each half wave plate is oriented so as to convert S polarized light to P polarized light and P polarized light to S polarized light.
In accordance with others of its aspects, the invention provides an optical system which comprises a polarization beam splitter of the type described above, a light source for providing light to a portion of the first side of the splitter, a projection lens for receiving light passing out of a portion of the second side of the splitter, said second side being opposite to the first side, and at least a first reflective, polarization converting, pixelized panel and a second reflective, polarization converting, pixelized panel, each panel serving to modulate light passing from the light source to the projection lens. In certain preferred embodiments, one or both of the pixelized panels are associated with sides of the splitter different from the sides of the splitter with which the light source and the projection lens are associated.
In accordance with further of its aspects, the invention provides a method for improving the contrast of a projection system employing at least one reflective, polarization converting, pixelized panel comprising passing light from a light source to the pixelized panel, modulating the light at the pixelized panel, and passing the modulated light to a projection lens, wherein:
(a) between the light source and the projection lens, at least some of the light passes through two half wave plates oriented so as to transform S polarized light to P polarized light and P polarized light to S polarized light;
(b) between the light source and the pixelized panel, the light interacts with two polarization diagonals, the light having S polarization for one of said interactions and having P polarization for the other of said interactions; and
(c) between the pixelized panel and the projection lens, the light interacts with two polarization diagonals, the light having S polarization for one of said interactions and having P polarization for the other of said interactions;
where an interaction with a polarization diagonal involves:
(i) primarily transmission through the diagonal if the light has P polarization; and
(ii) primarily reflection from the diagonal if the light has S polarization.
In accordance with certain preferred embodiments of these aspects, the invention provides a method for improving the contrast of a projection system employing at least two reflective, polarization converting, pixelized panels comprising passing light from a light source to the two pixelized panels, modulating the light at the two pixelized panels, and passing the modulated light to a projection lens, wherein:
(a) between the light source and the projection lens, a first portion of the light passes through first and second half wave plates and a second portion of the light passes through third and fourth half wave plates, each half wave plate being oriented so as to transform S polarized light to P polarized light and P polarized light to S polarized light;
(b) between the light source and each of the two pixelized panels, the light interacts with two polarization diagonals, the light having S polarization for one of said interactions and having P polarization for the other of said interactions; and
(c) between each of the two pixelized panels and the projection lens, the light interacts with two polarization diagonals, the light having S polarization for one of said interactions and having P polarization for the other of said interactions;
where an interaction with a polarization diagonal involves:
(i) primarily transmission through the diagonal if the light has P polarization; and
(ii) primarily reflection from the diagonal if the light has S polarization.
In accordance with still further of its aspects, the invention provides an optical system comprising:
(a) a polarization beam splitter for producing two beams of polarized light;
(b) a dichroic prism which receives one of said beams;
(c) an optical path length compensator which receives the other of said beams; and
(d) at least one light filter associated with the optical path length compensator.
In certain embodiments, the at least one light filter comprises two light filters on opposite sides of the compensator, one being a short pass filter, i.e., a filter which passes shorter wavelengths, and the other being a long pass filter, i.e., a filter which passes longer wavelengths.
As discussed below in detail, the invention achieves various benefits including cost reduction through the use of lower cost dichroics and/or filters for color separation, brightness enhancement through at least partial and, in some embodiments, complete polarization recovery, and high contrast between xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d pixels in the light transmitted to the viewing screen.